A heat transfer coil of the type used in air conditioning and refrigeration systems typically includes a plurality of refrigerant-carrying tubes and a plurality of fins extending between the tubes to enhance the heat transfer between the refrigerant in the tubes and the air passing through the coil. The tubes are typically made of aluminum, copper or stainless steel material. The fins are typically made of thin sheets of aluminum, copper, stainless steel or carbon steel material.
Coil manufacturing usually involves one of the following two processes: (1) application of petroleum-based lubricant to the fin stock prior to the fin stamping operation and to the tubes prior to both the bending and expanding operations, followed by application of a solvent-based vapor degreaser or an aqueous-based cleaning solution to remove lubricant residue; or (2) application of an evaporative lubricant containing approximately 90% mineral spirits and 10% petroleum additives to the fin stock before the fin stamping operation and applying a petroleum-based lubricant to the tubes prior to the tube bending operation.
The aforementioned first process, using a petroleum-based fin lubricant, involves the following steps: (a) continuous coil stock fin material (e.g., aluminum, copper, stainless steel or carbon steel raw material of 0.0045 to 0.0160 inch thickness) is fed into a fin press from an unreeler; (b) the fin material is passed through an oil bath filled with a petroleum-based lubricant and then through a pair of variable pressure rollers, which remove excess lubricant from the fin material; (c) the fin material is then worked through a fin press and die to form the desired fin configuration and fin density; (d) the tube material (e.g., copper, aluminum or stainless steel material) is fed in straight lengths into a tube bender where a petroleum-based lubricant is applied inside each tube length and each tube length is bent to form a U-shaped hairpin; (e) the hairpins are laced through openings in the fin collars to form a fin pack; (f) a petroleum-based lubricant is applied inside each hairpin and the hairpins are expanded by an expander tip to the desired diameter (e.g., 0.004 to 0.008 inch larger than the diameter prior to expansion), thereby "locking" the hairpins into the fin pack to form the coil; (g) the coil is then routed through a centralized, solvent-based vapor degreaser or, alternatively, through an aqueous-based cleaning system to clean both the fins and hairpins internally as well as externally; (h) return tube bends, distributors and headers are brazed to the coil in order to complete the coil assembly; (i) the assembled coil is leak tested, dried with blown air and routed through a dehydration oven; (j) the coil may be painted; and (k) the coil is purged internally with dry air to remove moisture and after the open tubes are sealed, the coil is boxed and ready for shipping.
The above-described first process, using a petroleum-based fin lubricant, has several disadvantages. One disadvantage is that some of the solvents used to remove the petroleum-based lubricant are designated by the Environmental Protection Agency (EPA) as suspected ozone depleaters. As such, certain control safeguards must be implemented and continually maintained in order to safely control vapors emitted from the solvent. Further, the solvents have a low flash point and are expensive to use because of the increased air quality regulations. Another disadvantage is that some petroleum-based lubricants applied to the fin stock are suspected as being carcinogenic with prolonged exposure to the skin and its discharge into water systems is strictly regulated. The use of aqueous-based cleaning systems instead of a solvent-based degreaser requires on-site filtration and waste water treatment, special disposal of filters and solids, and expensive modification of systems using solvent-based vapor degreasers. Due to the expense of implementing a cleaning system to remove lubricant residue from the fins and coils, it is not practical to operate a solvent-based vapor degreaser for each fin press line. As such, all assembled coils must be cleaned using a centralized cleaning system, which creates a bottleneck in the manufacturing process.
The aforementioned second process, using an evaporative fin lubricant, is normally used in facilities with a relatively small number (ten or fewer) of fin presses. This second process includes the following steps: (a) continuous coil stock fin material is fed from an unreeler through an oil bath filled with an evaporative lubricant and then through a pair of variable pressure rollers to remove excess lubricant; (b) evaporative lubricant is also applied through specially-designed die sections of the fin press for a period of about five seconds prior to operating the fin press and also during fin press operation; (c) the fin material is worked through the fin press and dies to form the fins with the desired fin configuration and fin density; (d) a petroleum-based lubricant is applied inside each straight length of tube at the location where each tube length is to be bent and the tube lengths are bent to form respective hairpins, as previously described; (e) the hairpins are laced through openings in the fin collars to form a fin pack; (f) the hairpins are expanded to the desired diameter with expander tips to form the coil (a petroleum-based lubricant is not applied to the hairpins during the expanding operation); (g) the assembled coils are stacked and staged for brazing, during which time the evaporative lubricant substantially evaporates, but the 10% petroleum-based additive remains on the fins; (h) return bends, distributors and headers are brazed to the coil to complete the coil assembly; (i) the assembled coil is leak tested, air blown, dehydrated, painted and dry air purged, and the open tubes are sealed, followed by boxing and shipping of the assembled coil.
This second process, using an evaporative lubricant, has several disadvantages. One disadvantage is that the volatile organic compound (VOC) emissions from the evaporative lubricant require a special permit from the EPA. Because of the permit requirement, the use of evaporative lubricants is not practical when there are a large number (i.e., more than ten) fin presses in a facility. Other disadvantages are that evaporative lubricants emit noxious vapors in operator breathing zones; the evaporative lubricants are petroleum-based and therefore subject to stringent disposal regulations; the use of evaporative lubricants results in diminished tooling life compared to non-evaporative lubricants because evaporative lubricants are comprised of approximately 10% petroleum, diluted with approximately 90% mineral spirits. The relatively poor tooling performance results in increased fin and die maintenance with related expenses and production downtime. Further, the fin press dies and controls must be specially designed to allow application of the evaporative lubricant through the dies, requiring fin press dwell prior to operation of the press, and the quality of fins produced with evaporative lubricants is typically inferior, which makes it more difficult to lace the hairpins through the fins during coil assembly. Also, a slight oil residue is left on the fins behind the coil header plate after the brazing operation. The major advantage of using evaporative lubricants is that a separate cleaning/degreasing step is not required.
A synthetic, non-petroleum-based fin lubricant has been used in lieu of petroleum-based lubricants in a coil manufacturing process. Various techniques may be used for applying the synthetic lubricant. One such technique is the conventional bath, whereby the fin material is immersed in the synthetic lubricant prior to entering the fin press. Although the conventional bath system is relatively inexpensive, an excessive amount of the synthetic lubricant is applied to the fin material. Although the variable pressure rollers are effective in removing some of the excess lubricant, the residue is still beyond acceptable limits. Further, the amount of lubricant applied to the fin material is a function of the viscosity of the lubricant. The greater the viscosity, the greater is the amount of lubricant applied to the fin material. To alleviate this problem, a recirculating heating system may be used to reduce the viscosity of the lubricant and therefore decrease the amount applied to the fin material. However, even with a recirculating heating system, the lubricant residue exceeds acceptable limits in the leak test tank and dehydration oven.
Another technique used to apply a synthetic lubricant to the fin material in a coil manufacturing process is an air-type spray system using a plurality of nozzles to atomize the synthetic lubricant applied to the fin material. Although the air-type spray system is effective in reducing the quantity of lubricant applied to the fin material, the system generates a large quantity of lubricant mist in the operators' breathing zones. To protect the operators, a vacuum system with an enclosure around the nozzles is required in order to contain the mist. This vacuum system takes up valuable floor space and requires regular maintenance, in addition to the initial expense.
Yet another technique for applying a synthetic lubricant to the fin material in a coil manufacturing process is an airless spray system, which has the advantage of reducing the quantity of lubricant mist in the operators' breathing zones. As such, a vacuum system is not required to contain the mist. Airless spray systems utilize air-assisted pistons which supply a metered volume of lubricant by hydraulic pressure to a nozzle, which atomizes the lubricant into a spray pattern. Among the disadvantages of the airless spray system are that each nozzle requires a dedicated hydraulic pump, which increases the likelihood of system failures and other related problems, particularly with the relatively large number of pumps and nozzles required for each fin press. If one of the nozzles fails to operate properly, part of the fin material may not receive the proper amount of lubricant, which can result in damage to the fin press and production downtime.
There is, therefore, a need for an improved process of manufacturing heat transfer coils, which does not require a separate cleaning step to remove lubricant residue from the tubes and fins of the coil and which is characterized by sufficiently low VOC emissions that special EPA permits are not required. There is also a need for an improved method of applying a synthetic, non-petroleum-based lubricant to the fin material in a coil manufacturing process.